Methods of reducing cd loss in a microelectromechanical device

ABSTRACT

Methods of fabricating an electromechanical systems device that minimize critical dimension (CD) loss in the device are described. The methods provide electromechanical systems devices with improved properties, including high reflectivity.

BACKGROUND Field of the Invention

Microelectromechanical systems (MEMS) include micro mechanical elements,actuators, and electronics. Micromechanical elements may be createdusing deposition, etching, and or other micromachining processes thatetch away parts of substrates and/or deposited material layers or thatadd layers to form electrical and electromechanical devices. One type ofMEMS device is called an interferometric modulator. As used herein, theterm interferometric modulator or interferometric light modulator refersto a device that selectively absorbs and/or reflects light using theprinciples of optical interference. In certain embodiments, aninterferometric modulator may comprise a pair of conductive plates, oneor both of which may be transparent and/or reflective in whole or partand capable of relative motion upon application of an appropriateelectrical signal. In a particular embodiment, one plate may comprise astationary layer deposited on a substrate and the other plate maycomprise a metallic membrane separated from the stationary layer by anair gap. As described herein in more detail, the position of one platein relation to another can change the optical interference of lightincident on the interferometric modulator. Such devices have a widerange of applications, and it would be beneficial in the art to utilizeand/or modify the characteristics of these types of devices so thattheir features can be exploited in improving existing products andcreating new products that have not yet been developed.

SUMMARY

The system, method, and devices of the invention each have severalaspects, no single one of which is solely responsible for its desirableattributes. Without limiting the scope of this invention, its moreprominent features will now be discussed briefly. After considering thisdiscussion, and particularly after reading the section entitled“Detailed Description of Preferred Embodiments” one will understand howthe features of this invention provide advantages over other displaydevices.

One aspect provides a method of fabricating an electromechanical systemsdevice that includes providing a sacrificial material and forming afirst metal layer over the sacrificial material. The first metal layercan be etched to form at least one first opening in the first metallayer, and the at least one first opening exposes a first surface of thesacrificial material. The method of fabricating an electromechanicalsystems device can include forming a second layer over the first surfaceof the sacrificial material. Typically, the second layer of material hasa smaller thickness dimension than the first metal layer. The secondlayer can then be etched to form at least one second opening in thesecond layer. In an embodiment, the second opening exposes at least aportion of the first surface of the sacrificial material and has asmaller dimension than the first opening.

Another aspect provides a method for fabricating an electromechanicalsystems device that includes providing a sacrificial material andforming a first metal layer over the sacrificial material. The firstmetal layer can be etched to form at least one opening in the firstmetal layer, and the at least one opening in the first metal layerexposes a first surface area of the sacrificial material. The method offabricating an electromechanical systems device can include forming asecond layer over the first surface area of the sacrificial material.Typically, the second layer has a smaller thickness dimension than thefirst metal layer. A first masking layer can be formed over the secondlayer and over a portion of the first surface area of the sacrificialmaterial, thereby forming an unmasked portion of the second layer overthe first surface area. The method can then include etching the unmaskedportion of the second layer to form at least one opening in the secondlayer. The at least one opening in the second layer exposes a secondsurface area of the sacrificial material. In an embodiment, theresulting exposed second surface area of the sacrificial material issmaller than the first surface area of the sacrificial material.Afterward, the first masking layer can optionally be removed.

Another aspect provides an interferometric modulator including asubstrate, an optical layer patterned into rows, a mechanical layerpatterned into columns, and a mirror layer separated from the opticallayer by a vertical gap. The mirror layer can include a plurality ofmirrors, and the thickness of the mirror layer can be greater than orequal to about 0.75 microns. The plurality of mirrors are separated fromone another by a horizontal gap that is less than or equal to about 7microns.

Another aspect provides an electromechanical device that includes amirror. The mirror can include a core portion having an exposedreflective surface and an overlaying mirror extension portion having anexposed reflective surface. The exposed reflective surfaces of the coreportion and the mirror extension portion can be co-planar.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view depicting a portion of one embodiment of aninterferometric modulator display in which a movable reflective layer ofa first interferometric modulator is in a relaxed position and a movablereflective layer of a second interferometric modulator is in an actuatedposition.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device incorporating a 3×3 interferometric modulator display.

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.

FIG. 4 is an illustration of a set of row and column voltages that maybe used to drive an interferometric modulator display.

FIGS. 5A and 5B illustrate one exemplary timing diagram for row andcolumn signals that may be used to write a frame of display data to the3×3 interferometric modulator display of FIG. 2.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa visual display device comprising a plurality of interferometricmodulators.

FIG. 7A is a cross section of the device of FIG. 1.

FIG. 7B is a cross section of an alternative embodiment of aninterferometric modulator.

FIG. 7C is a cross section of another alternative embodiment of aninterferometric modulator.

FIG. 7D is a cross section of yet another alternative embodiment of aninterferometric modulator.

FIG. 7E is a cross section of an additional alternative embodiment of aninterferometric modulator.

FIG. 8 is a flow diagram illustrating certain steps in an embodiment ofa method of making an interferometric modulator.

FIG. 9 is a graph depicting the percentage reduction of brightness as afunction of pixel size for different levels of mirror CD loss per edge.

FIG. 10 is a graph depicting the temperature warp sensitivity of amirror as a function of the thickness of the mirror.

FIGS. 11A through 11C illustrate the CD loss that occurs during themanufacture of a MEMS device according to previously known methods.

FIGS. 12A through 12E show an embodiment of the processing steps ofmanufacturing a MEMS device.

FIGS. 13A through 13F show another embodiment of the processing steps ofmanufacturing a MEMS device.

FIG. 14 is an embodiment of a MEMS device having a reflective layer withtapered edges.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The following detailed description is directed to certain specificembodiments. However, the teachings herein can be applied in a multitudeof different ways. In this description, reference is made to thedrawings wherein like parts are designated with like numeralsthroughout. The embodiments may be implemented in any device that isconfigured to display an image, whether in motion (e.g., video) orstationary (e.g., still image), and whether textual or pictorial. Moreparticularly, it is contemplated that the embodiments may be implementedin or associated with a variety of electronic devices such as, but notlimited to, mobile telephones, wireless devices, personal dataassistants (PDAs), hand-held or portable computers, GPSreceivers/navigators, cameras, MP3 players, camcorders, game consoles,wrist watches, clocks, calculators, television monitors, flat paneldisplays, computer monitors, auto displays (e.g., odometer display,etc.), cockpit controls and/or displays, display of camera views (e.g.,display of a rear view camera in a vehicle), electronic photographs,electronic billboards or signs, projectors, architectural structures,packaging, and aesthetic structures (e.g., display of images on a pieceof jewelry). MEMS devices of similar structure to those described hereincan also be used in non-display applications such as in electronicswitching devices.

In many MEMS devices (including certain interferometric modulators), themechanical properties are controlled by the design and geometry of themechanical layer and the optical properties are separately controlled bythe mirror layer. One desirable optical property of the mirror layer inMEMS devices, as perceived by the end user, is brightness. The level ofbrightness can be increased by providing a mirror layer with a largesurface area. However, some of the manufacturing steps used in thecreation of MEMS devices, including etching, may generate erosion at theouter edge of the mirror layer resulting in critical dimension (CD)loss, ultimately decreasing the surface area of the mirror layers.Desirably, the mirror layer in a MEMS device is provided with arelatively large mirror thickness, such that the mirror has sufficientrigidity to maintain flatness over a wide temperature range. However, asthe thickness of the mirror increases, a greater degree of etching isoften desired in the manufacture of the MEMS device, which furtherincreases the CD loss. Greater CD loss can be attributed to theisotropic nature and inherent aspect ratio of an etchant's etchingprofile in that an etchant necessarily removes material extending out ina horizontal direction as it removes material in the vertical direction.As a result, longer etching times or etchants with higher etching ratesthat are needed to penetrate thicker material layers allow an etchant toetch farther in the horizontal direction and thereby increase CD loss.Accordingly, in some embodiments described herein, methods ofmanufacturing a MEMS device are provided that result in low CD loss tothe mirror layer. The manufacturing processes described herein may beused to provide MEMS devices with both high reflectivity and highrigidity in the mirror layers.

One interferometric modulator display embodiment comprising aninterferometric MEMS display element is illustrated in FIG. 1. In thesedevices, the pixels are in either a bright or dark state. In the bright(“relaxed” or “open”) state, the display element reflects a largeportion of incident visible light to a user. When in the dark(“actuated” or “closed”) state, the display element reflects littleincident visible light to the user. Depending on the embodiment, thelight reflectance properties of the “on” and “off” states may bereversed. MEMS pixels can be configured to reflect predominantly atselected colors, allowing for a color display in addition to black andwhite.

FIG. 1 is an isometric view depicting two adjacent pixels in a series ofpixels of a visual display, wherein each pixel comprises a MEMSinterferometric modulator. In some embodiments, an interferometricmodulator display comprises a row/column array of these interferometricmodulators. Each interferometric modulator includes a pair of reflectivelayers positioned at a variable and controllable distance from eachother to form a resonant optical gap with at least one variabledimension. In one embodiment, one of the reflective layers may be movedbetween two positions. In the first position, referred to herein as therelaxed position, the movable reflective layer is positioned at arelatively large distance from a fixed partially reflective layer. Inthe second position, referred to herein as the actuated position, themovable reflective layer is positioned more closely adjacent to thepartially reflective layer. Incident light that reflects from the twolayers interferes constructively or destructively depending on theposition of the movable reflective layer, producing either an overallreflective or non-reflective state for each pixel.

The depicted portion of the pixel array in FIG. 1 includes two adjacentinterferometric modulators 12 a and 12 b. In the interferometricmodulator 12 a on the left, a movable reflective layer 14 a isillustrated in a relaxed position at a predetermined distance from anoptical stack 16 a, which includes a partially reflective layer. In theinterferometric modulator 12 b on the right, the movable reflectivelayer 14 b is illustrated in an actuated position adjacent to theoptical stack 16 b.

The optical stacks 16 a and 16 b (collectively referred to as opticalstack 16), as referenced herein, typically comprise several fusedlayers, which can include an electrode layer, such as indium tin oxide(ITO), a partially reflective layer, such as chromium, and a transparentdielectric. The optical stack 16 is thus electrically conductive,partially transparent and partially reflective, and may be fabricated,for example, by depositing one or more of the above layers onto atransparent substrate 20. The partially reflective layer can be formedfrom a variety of materials that are partially reflective such asvarious metals, semiconductors, and dielectrics. The partiallyreflective layer can be formed of one or more layers of materials, andeach of the layers can be formed of a single material or a combinationof materials.

In some embodiments, the layers of the optical stack 16 are patternedinto parallel strips, and may form row electrodes in a display device asdescribed further below. The movable reflective layers 14 a, 14 b may beformed as a series of parallel strips of a deposited metal layer orlayers (orthogonal to the row electrodes of 16 a, 16 b) to form columnsdeposited on top of posts 18 and an intervening sacrificial materialdeposited between the posts 18. When the sacrificial material is etchedaway, the movable reflective layers 14 a, 14 b are separated from theoptical stacks 16 a, 16 b by a defined gap 19. A highly conductive andreflective material such as aluminum may be used for the reflectivelayers 14, and these strips may form column electrodes in a displaydevice. Note that FIG. 1 may not be to scale. In some embodiments, thespacing between posts 18 may be on the order of 10-100 um, while the gap19 may be on the order of <1000 Angstroms.

With no applied voltage, the gap 19 remains between the movablereflective layer 14 a and optical stack 16 a, with the movablereflective layer 14 a in a mechanically relaxed state, as illustrated bythe pixel 12 a in FIG. 1. However, when a potential (voltage) differenceis applied to a selected row and column, the capacitor formed at theintersection of the row and column electrodes at the corresponding pixelbecomes charged, and electrostatic forces pull the electrodes together.If the voltage is high enough, the movable reflective layer 14 isdeformed and is forced against the optical stack 16. A dielectric layer(not illustrated in this Figure) within the optical stack 16 may preventshorting and control the separation distance between layers 14 and 16,as illustrated by actuated pixel 12 b on the right in FIG. 1. Thebehavior is the same regardless of the polarity of the applied potentialdifference.

FIGS. 2 through 5 illustrate one exemplary process and system for usingan array of interferometric modulators in a display application.

FIG. 2 is a system block diagram illustrating one embodiment of anelectronic device that may incorporate interferometric modulators. Theelectronic device includes a processor 21 which may be any generalpurpose single- or multi-chip microprocessor such as an ARM®, Pentium®,8051, MIPS®, Power PC®, or ALPHA®, or any special purpose microprocessorsuch as a digital signal processor, microcontroller, or a programmablegate array. As is conventional in the art, the processor 21 may beconfigured to execute one or more software modules. In addition toexecuting an operating system, the processor may be configured toexecute one or more software applications, including a web browser, atelephone application, an email program, or any other softwareapplication.

In one embodiment, the processor 21 is also configured to communicatewith an array driver 22. In one embodiment, the array driver 22 includesa row driver circuit 24 and a column driver circuit 26 that providesignals to a display array or panel 30. The cross section of the arrayillustrated in FIG. 1 is shown by the lines 1-1 in FIG. 2. Note thatalthough FIG. 2 illustrates a 3×3 array of interferometric modulatorsfor the sake of clarity, the display array 30 may contain a very largenumber of interferometric modulators, and may have a different number ofinterferometric modulators in rows than in columns (e.g., 300 pixels perrow by 190 pixels per column).

FIG. 3 is a diagram of movable mirror position versus applied voltagefor one exemplary embodiment of an interferometric modulator of FIG. 1.For MEMS interferometric modulators, the row/column actuation protocolmay take advantage of a hysteresis property of these devices asillustrated in FIG. 3. An interferometric modulator may require, forexample, a 10 volt potential difference to cause a movable layer todeform from the relaxed state to the actuated state. However, when thevoltage is reduced from that value, the movable layer maintains itsstate as the voltage drops back below 10 volts. In the exemplaryembodiment of FIG. 3, the movable layer does not relax completely untilthe voltage drops below 2 volts. There is thus a range of voltage, about3 to 7 V in the example illustrated in FIG. 3, where there exists awindow of applied voltage within which the device is stable in eitherthe relaxed or actuated state. This is referred to herein as the“hysteresis window” or “stability window.” For a display array havingthe hysteresis characteristics of FIG. 3, the row/column actuationprotocol can be designed such that during row strobing, pixels in thestrobed row that are to be actuated are exposed to a voltage differenceof about 10 volts, and pixels that are to be relaxed are exposed to avoltage difference of close to zero volts. After the strobe, the pixelsare exposed to a steady state or bias voltage difference of about 5volts such that they remain in whatever state the row strobe put themin. After being written, each pixel sees a potential difference withinthe “stability window” of 3-7 volts in this example. This feature makesthe pixel design illustrated in FIG. 1 stable under the same appliedvoltage conditions in either an actuated or relaxed pre-existing state.Since each pixel of the interferometric modulator, whether in theactuated or relaxed state, is essentially a capacitor formed by thefixed and moving reflective layers, this stable state can be held at avoltage within the hysteresis window with almost no power dissipation.Essentially no current flows into the pixel if the applied potential isfixed.

As described further below, in typical applications, a frame of an imagemay be created by sending a set of data signals (each having a certainvoltage level) across the set of column electrodes in accordance withthe desired set of actuated pixels in the first row. A row pulse is thenapplied to a first row electrode, actuating the pixels corresponding tothe set of data signals. The set of data signals is then changed tocorrespond to the desired set of actuated pixels in a second row. Apulse is then applied to the second row electrode, actuating theappropriate pixels in the second row in accordance with the datasignals. The first row of pixels are unaffected by the second row pulse,and remain in the state they were set to during the first row pulse.This may be repeated for the entire series of rows in a sequentialfashion to produce the frame. Generally, the frames are refreshed and/orupdated with new image data by continually repeating this process atsome desired number of frames per second. A wide variety of protocolsfor driving row and column electrodes of pixel arrays to produce imageframes may be used.

FIGS. 4 and 5 illustrate one possible actuation protocol for creating adisplay frame on the 3×3 array of FIG. 2. FIG. 4 illustrates a possibleset of column and row voltage levels that may be used for pixelsexhibiting the hysteresis curves of FIG. 3. In the FIG. 4 embodiment,actuating a pixel involves setting the appropriate column to −V_(bias),and the appropriate row to +ΔV, which may correspond to −5 volts and +5volts respectively Relaxing the pixel is accomplished by setting theappropriate column to +V_(bias), and the appropriate row to the same+ΔV, producing a zero volt potential difference across the pixel. Inthose rows where the row voltage is held at zero volts, the pixels arestable in whatever state they were originally in, regardless of whetherthe column is at +V_(bias), or −V_(bias). As is also illustrated in FIG.4, voltages of opposite polarity than those described above can be used,e.g., actuating a pixel can involve setting the appropriate column to+V_(bias), and the appropriate row to −ΔV. In this embodiment, releasingthe pixel is accomplished by setting the appropriate column to−V_(bias), and the appropriate row to the same −ΔV, producing a zerovolt potential difference across the pixel.

FIG. 5B is a timing diagram showing a series of row and column signalsapplied to the 3×3 array of FIG. 2 which will result in the displayarrangement illustrated in FIG. 5A, where actuated pixels arenon-reflective. Prior to writing the frame illustrated in FIG. 5A, thepixels can be in any state, and in this example, all the rows areinitially at 0 volts, and all the columns are at +5 volts. With theseapplied voltages, all pixels are stable in their existing actuated orrelaxed states.

In the FIG. 5A frame, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) areactuated. To accomplish this, during a “line time” for row 1, columns 1and 2 are set to −5 volts, and column 3 is set to +5 volts. This doesnot change the state of any pixels, because all the pixels remain in the3-7 volt stability window. Row 1 is then strobed with a pulse that goesfrom 0, up to 5 volts, and back to zero. This actuates the (1,1) and(1,2) pixels and relaxes the (1,3) pixel. No other pixels in the arrayare affected. To set row 2 as desired, column 2 is set to −5 volts, andcolumns 1 and 3 are set to +5 volts. The same strobe applied to row 2will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again,no other pixels of the array are affected. Row 3 is similarly set bysetting columns 2 and 3 to −5 volts, and column 1 to +5 volts. The row 3strobe sets the row 3 pixels as shown in FIG. 5A. After writing theframe, the row potentials are zero, and the column potentials can remainat either +5 or −5 volts, and the display is then stable in thearrangement of FIG. 5A. The same procedure can be employed for arrays ofdozens or hundreds of rows and columns. The timing, sequence, and levelsof voltages used to perform row and column actuation can be variedwidely within the general principles outlined above, and the aboveexample is exemplary only, and any actuation voltage method can be usedwith the systems and methods described herein.

FIGS. 6A and 6B are system block diagrams illustrating an embodiment ofa display device 40. The display device 40 can be, for example, acellular or mobile telephone. However, the same components of displaydevice 40 or slight variations thereof are also illustrative of varioustypes of display devices such as televisions and portable media players.

The display device 40 includes a housing 41, a display 30, an antenna43, a speaker 45, an input device 48, and a microphone 46. The housing41 is generally formed from any of a variety of manufacturing processes,including injection molding, and vacuum forming. In addition, thehousing 41 may be made from any of a variety of materials, including butnot limited to plastic, metal, glass, rubber, and ceramic, or acombination thereof. In one embodiment the housing 41 includes removableportions (not shown) that may be interchanged with other removableportions of different color, or containing different logos, pictures, orsymbols.

The display 30 of exemplary display device 40 may be any of a variety ofdisplays, including a bi-stable display, as described herein. In otherembodiments, the display 30 includes a flat-panel display, such asplasma, EL, OLED, STN LCD, or TFT LCD as described above, or anon-flat-panel display, such as a CRT or other tube device. However, forpurposes of describing the present embodiment, the display 30 includesan interferometric modulator display, as described herein.

The components of one embodiment of exemplary display device 40 areschematically illustrated in FIG. 6B. The illustrated exemplary displaydevice 40 includes a housing 41 and can include additional components atleast partially enclosed therein. For example, in one embodiment, theexemplary display device 40 includes a network interface 27 thatincludes an antenna 43 which is coupled to a transceiver 47. Thetransceiver 47 is connected to a processor 21, which is connected toconditioning hardware 52. The conditioning hardware 52 may be configuredto condition a signal (e.g. filter a signal). The conditioning hardware52 is connected to a speaker 45 and a microphone 46. The processor 21 isalso connected to an input device 48 and a driver controller 29. Thedriver controller 29 is coupled to a frame buffer 28, and to an arraydriver 22, which in turn is coupled to a display array 30. A powersupply 50 provides power to all components as required by the particularexemplary display device 40 design.

The network interface 27 includes the antenna 43 and the transceiver 47so that the exemplary display device 40 can communicate with one or moredevices over a network. In one embodiment the network interface 27 mayalso have some processing capabilities to relieve requirements of theprocessor 21. The antenna 43 is any antenna for transmitting andreceiving signals. In one embodiment, the antenna transmits and receivesRF signals according to the IEEE 802.11 standard, including IEEE802.11(a), (b), or (g). In another embodiment, the antenna transmits andreceives RF signals according to the BLUETOOTH standard. In the case ofa cellular telephone, the antenna is designed to receive CDMA, GSM,AMPS, W-CDMA, or other known signals that are used to communicate withina wireless cell phone network. The transceiver 47 pre-processes thesignals received from the antenna 43 so that they may be received by andfurther manipulated by the processor 21. The transceiver 47 alsoprocesses signals received from the processor 21 so that they may betransmitted from the exemplary display device 40 via the antenna 43.

In an alternative embodiment, the transceiver 47 can be replaced by areceiver. In yet another alternative embodiment, network interface 27can be replaced by an image source, which can store or generate imagedata to be sent to the processor 21. For example, the image source canbe a digital video disc (DVD) or a hard-disc drive that contains imagedata, or a software module that generates image data.

Processor 21 generally controls the overall operation of the exemplarydisplay device 40. The processor 21 receives data, such as compressedimage data from the network interface 27 or an image source, andprocesses the data into raw image data or into a format that is readilyprocessed into raw image data. The processor 21 then sends the processeddata to the driver controller 29 or to frame buffer 28 for storage. Rawdata typically refers to the information that identifies the imagecharacteristics at each location within an image. For example, suchimage characteristics can include color, saturation, and gray-scalelevel.

In one embodiment, the processor 21 includes a microcontroller, CPU, orlogic unit to control operation of the exemplary display device 40.Conditioning hardware 52 generally includes amplifiers and filters fortransmitting signals to the speaker 45, and for receiving signals fromthe microphone 46. Conditioning hardware 52 may be discrete componentswithin the exemplary display device 40, or may be incorporated withinthe processor 21 or other components.

The driver controller 29 takes the raw image data generated by theprocessor 21 either directly from the processor 21 or from the framebuffer 28 and reformats the raw image data appropriately for high speedtransmission to the array driver 22. Specifically, the driver controller29 reformats the raw image data into a data flow having a raster-likeformat, such that it has a time order suitable for scanning across thedisplay array 30. Then the driver controller 29 sends the formattedinformation to the array driver 22. Although a driver controller 29,such as a LCD controller, is often associated with the system processor21 as a stand-alone Integrated Circuit (IC), such controllers may beimplemented in many ways. They may be embedded in the processor 21 ashardware, embedded in the processor 21 as software, or fully integratedin hardware with the array driver 22.

Typically, the array driver 22 receives the formatted information fromthe driver controller 29 and reformats the video data into a parallelset of waveforms that are applied many times per second to the hundredsand sometimes thousands of leads coming from the display's x-y matrix ofpixels.

In one embodiment, the driver controller 29, array driver 22, anddisplay array 30 are appropriate for any of the types of displaysdescribed herein. For example, in one embodiment, driver controller 29is a conventional display controller or a bi-stable display controller(e.g., an interferometric modulator controller). In another embodiment,array driver 22 is a conventional driver or a bi-stable display driver(e.g., an interferometric modulator display). In one embodiment, adriver controller 29 is integrated with the array driver 22. Such anembodiment is common in highly integrated systems such as cellularphones, watches, and other small area displays. In yet anotherembodiment, display array 30 is a typical display array or a bi-stabledisplay array (e.g., a display including an array of interferometricmodulators).

The input device 48 allows a user to control the operation of theexemplary display device 40. In one embodiment, input device 48 includesa keypad, such as a QWERTY keyboard or a telephone keypad, a button, aswitch, a touch-sensitive screen, a pressure- or heat-sensitivemembrane. In one embodiment, the microphone 46 is an input device forthe exemplary display device 40. When the microphone 46 is used to inputdata to the device, voice commands may be provided by a user forcontrolling operations of the exemplary display device 40.

Power supply 50 can include a variety of energy storage devices as arewell known in the art. For example, in one embodiment, power supply 50is a rechargeable battery, such as a nickel-cadmium battery or a lithiumion battery. In another embodiment, power supply 50 is a renewableenergy source, a capacitor, or a solar cell, including a plastic solarcell, and solar-cell paint. In another embodiment, power supply 50 isconfigured to receive power from a wall outlet.

In some implementations control programmability resides, as describedabove, in a driver controller which can be located in several places inthe electronic display system. In some cases control programmabilityresides in the array driver 22. The above-described optimization may beimplemented in any number of hardware and/or software components and invarious configurations.

The details of the structure of interferometric modulators that operatein accordance with the principles set forth above may vary widely. Forexample, FIGS. 7A-7E illustrate five different embodiments of themovable reflective layer 14 and its supporting structures. FIG. 7A is across section of the embodiment of FIG. 1, where a strip of metalmaterial 14 is deposited on orthogonally extending supports 18. In FIG.7B, the moveable reflective layer 14 of each interferometric modulatoris square or rectangular in shape and attached to supports at thecorners only, on tethers 32. In FIG. 7C, the moveable reflective layer14 is square or rectangular in shape and suspended from a deformablelayer 34, which may comprise a flexible metal. The deformable layer 34connects, directly or indirectly, to the substrate 20 around theperimeter of the deformable layer 34. These connections are hereinreferred to as support posts. The embodiment illustrated in FIG. 7D hassupport post plugs 42 upon which the deformable layer 34 rests. Themovable reflective layer 14 remains suspended over the gap, as in FIGS.7A-7C, but the deformable layer 34 does not form the support posts byfilling holes between the deformable layer 34 and the optical stack 16.Rather, the support posts are formed of a planarization material, whichis used to form support post plugs 42. The embodiment illustrated inFIG. 7E is based on the embodiment shown in FIG. 7D, but may also beadapted to work with any of the embodiments illustrated in FIGS. 7A-7Cas well as additional embodiments not shown. In the embodiment shown inFIG. 7E, an extra layer of metal or other conductive material has beenused to form a bus structure 44. This allows signal routing along theback of the interferometric modulators, eliminating a number ofelectrodes that may otherwise have had to be formed on the substrate 20.

In embodiments such as those shown in FIG. 7, the interferometricmodulators function as direct-view devices, in which images are viewedfrom the front side of the transparent substrate 20, the side oppositeto that upon which the modulator is arranged. In these embodiments, thereflective layer 14 optically shields the portions of theinterferometric modulator on the side of the reflective layer oppositethe substrate 20, including the deformable layer 34. This allows theshielded areas to be configured and operated upon without negativelyaffecting the image quality. For example, such shielding allows the busstructure 44 in FIG. 7E, which provides the ability to separate theoptical properties of the modulator from the electromechanicalproperties of the modulator, such as addressing and the movements thatresult from that addressing. This separable modulator architectureallows the structural design and materials used for theelectromechanical aspects and the optical aspects of the modulator to beselected and to function independently of each other. Moreover, theembodiments shown in FIGS. 7C-7E have additional benefits deriving fromthe decoupling of the optical properties of the reflective layer 14 fromits mechanical properties, which are carried out by the deformable layer34. This allows the structural design and materials used for thereflective layer 14 to be optimized with respect to the opticalproperties, and the structural design and materials used for thedeformable layer 34 to be optimized with respect to desired mechanicalproperties.

The interferometric modulators described above may be manufactured usingany suitable manufacturing techniques known in the art for making MEMSdevices. For example, the various material layers making up theinterferometric modulators may be sequentially deposited onto atransparent substrate with appropriate patterning and etching stepsconducted between deposition steps. In some embodiments, multiple layersmay be deposited during interferometric modulator manufacturing withoutany etching steps between the deposition steps. For example, the movablereflective layer described above may comprise a composite structurehaving two or more layers.

FIG. 8 illustrates certain steps in an embodiment of a manufacturingprocess 800 for an interferometric modulator. Such steps may be presentin a process for manufacturing, e.g., interferometric modulators of thegeneral type illustrated in FIGS. 1 and 7, along with other steps notshown in FIG. 8. With reference to FIGS. 1, 7 and 8, the process 800begins at step 805 with the formation of the optical stack 16 over thesubstrate 20. The substrate 20 may be a transparent substrate such asglass or plastic and may have been subjected to prior preparationstep(s), e.g., cleaning, to facilitate efficient formation of theoptical stack 16. As discussed above, the optical stack 16 iselectrically conductive, partially transparent and partially reflective,and may be fabricated, for example, by depositing one or more of thelayers onto the transparent substrate 20. In some embodiments, thelayers are patterned into parallel strips, and may form row electrodesin a display device. In some embodiments, the optical stack 16 includesan insulating or dielectric layer that is deposited over one or moremetal layers (e.g., reflective and/or conductive layers).

The process 800 illustrated in FIG. 8 continues at step 810 with theformation of a sacrificial layer over the optical stack 16. Thesacrificial layer is later removed (e.g., at step 825) to form the gap19 as discussed below and thus the sacrificial layer is not shown in theresulting interferometric modulator 12 illustrated in FIGS. 1 and 7. Theformation of the sacrificial layer over the optical stack 16 may includedeposition of a XeF₂-etchable material such as molybdenum or amorphoussilicon, in a thickness selected to provide, after subsequent removal, agap 19 having the desired size. Deposition of the sacrificial materialmay be carried out using deposition techniques such as physical vapordeposition (PVD, e.g., sputtering), plasma-enhanced chemical vapordeposition (PECVD), thermal chemical vapor deposition (thermal CVD), orspin-coating.

The process 800 illustrated in FIG. 8 continues at step 815 with theformation of a support structure e.g., a post 18 as illustrated in FIGS.1 and 7. The formation of the post 18 may include the steps ofpatterning the sacrificial layer to form a support structure aperture,then depositing a material (e.g., a polymer) into the aperture to formthe post 18, using a deposition method such as PECVD, thermal CVD, orspin-coating. In some embodiments, the support structure aperture formedin the sacrificial layer extends through both the sacrificial layer andthe optical stack 16 to the underlying substrate 20, so that the lowerend of the post 18 contacts the substrate 20 as illustrated in FIG. 7A.In other embodiments, the aperture formed in the sacrificial layerextends through the sacrificial layer, but not through the optical stack16. For example, FIG. 7C illustrates the lower end of the support postplugs 42 in contact with the optical stack 16.

The process 800 illustrated in FIG. 8 continues at step 820 with theformation of a movable reflective layer such as the movable reflectivelayer 14 illustrated in FIGS. 1 and 7. The movable reflective layer 14may be formed by employing one or more deposition steps, e.g.,reflective layer (e.g., aluminum, aluminum alloy) deposition, along withone or more patterning, masking, and/or etching steps. As discussedabove, the movable reflective layer 14 is typically electricallyconductive, and may be referred to herein as an electrically conductivelayer. Since the sacrificial layer is still present in the partiallyfabricated interferometric modulator formed at step 820 of the process800, the movable reflective layer 14 is typically not movable at thisstage. A partially fabricated interferometric modulator that contains asacrificial layer may be referred to herein as an “unreleased”interferometric modulator.

The process 800 illustrated in FIG. 8 continues at step 825 with theformation of a gap, e.g., a gap 19 as illustrated in FIGS. 1 and 7. Thegap 19 may be formed by exposing the sacrificial material (deposited atstep 810) to an etchant. For example, an etchable sacrificial materialsuch as molybdenum or amorphous silicon may be removed by dry chemicaletching, e.g., by exposing the sacrificial layer to a gaseous orvaporous etchant, such as vapors derived from solid xenon difluoride(XeF₂) for a period of time that is effective to remove the desiredamount of material, typically selectively relative to the structuressurrounding the gap 19. Other etching methods, e.g. wet etching and/orplasma etching, may also be used. Since the sacrificial layer is removedduring step 825 of the process 800, the movable reflective layer 14 istypically movable after this stage. After removal of the sacrificialmaterial, the resulting fully or partially fabricated interferometricmodulator may be referred to herein as a “released” interferometricmodulator.

Brightness is a desirable characteristic in a number of MEMS devices.While the size of the pixels within an optical MEMS device, such as aninterferometric modulator, has some effect on the brightness of thedevice, the amount of CD loss in the mirror layers during themanufacture of the MEMS device is often a greater factor in its effecton the brightness than pixel size. For example, FIG. 9 depicts thepercentage reduction of brightness as a function of the pixel size fordifferent levels of mirror CD loss per edge for an embodiment of aninterferometric modulator. The graph in FIG. 9 shows that whileincreasing the pixel size provides a gradual trend of increasedbrightness, higher levels of brightness are achieved when CD loss of themirror layer is minimized. For every 1 μm of CD loss per edge of themirror in this embodiment, there is approximately a 2.5% drop inbrightness.

Mirror thickness is related to the ability to control the flatness ofthe mirror over a large temperature range. Using mirrors withinsufficient thickness in an optical MEMS device can result in mirrorcurvature over time and temperature. FIG. 10 depicts the temperaturewarp sensitivity of a mirror as a function of the thickness of themirror for an embodiment of an interferometric modulator. As thethickness of the mirror increases, the warp sensitivity of the mirrordecreases. Mirrors in this embodiment are less likely to warp as thethickness increases up to 1.5 microns, preferably up to 2 microns.Therefore, a mirror layer is more likely to remain flat over a largertemperature range when the thickness of the mirror is increased.

While increasing mirror thickness is desirable, doing so may involvemore rigorous etching conditions during the fabrication of the MEMSdevice. The rigorous etching conditions have the consequence ofgenerating greater amounts of CD loss in the mirror layer. CD loss isincreased when the horizontal gap between adjacent mirrors in a mirrorlayer is widened upon etching the mirror layer. By using a thinnermirror layer in the fabrication steps of the MEMS device, the mirrorlayer is less exposed to etching conditions and CD loss is minimized.

FIGS. 11A through 11C illustrate the CD loss that occurs during themanufacture of a typical mirror layer in a MEMS device according topreviously known methods. The structure 100 illustrated in FIG. 11A canbe used to form an interferometric modulator, for example, aninterferometric modulator of the general type illustrated in FIGS. 1 and7. For example, the bottom layer 101 in FIG. 11 can correspond to thesubstrate layer 20 illustrated in FIGS. 1 and 7 and the upper layer 104in FIG. 11 can correspond to the reflective layer 14 illustrated inFIGS. 1 and 7. Although not illustrated in FIG. 11, additional layersand materials that may be present in an interferometric modulator (e.g.optical stack 16, deformable layer 34, etc.) can also be present in themethod illustrated by FIG. 11.

As illustrated in FIG. 11A, the initial steps of fabricating a MEMSdevice may involve forming a structure 100 by forming a sacrificiallayer 102 on a substrate 101, and then depositing a first metal layer104 onto the sacrificial layer 102. As illustrated in FIG. 11B, thefirst metal layer 104 is then chemically etched using an etchant thatstops on the sacrificial layer 102. The etching step causes theformation of an opening 105 (which corresponds to the CD loss in themetal layer) at the edges 106 of the metal layer 104. In FIG. 11C, thesacrificial layer 102 is removed (e.g. to form a gap 109 underneath themetal layer 104). Support posts (not illustrated), e.g. support posts 18as represented in FIGS. 1 and 7, can be put in place to maintain thespacing of the gap 109 between the substrate 101 and the first metallayer 104. The gap 109 in FIG. 11C can correspond to the gap 19 inFIG. 1. The surface area and reflectivity of the first metal layer 104is significantly lessened due to the CD loss of opening 105 and thebrightness level of the resulting MEMS device, as perceived by the enduser, is lower. The CD loss of opening 105 is even greater when athicker metal layer 104 is used corresponding to a number of factors.For example, as discussed above, longer etching times and/or higheretching rate etchants are often required with thicker metal layers.

Disclosed herein are methods of fabricating a MEMS device that allow forthe use of a relatively thick mirror layer and also minimize the CD lossin the mirror layer during MEMS device fabrication. FIGS. 12A through12E show an embodiment of the processing steps of manufacturing a MEMSdevice. The structure 150 illustrated in FIG. 12 can be used to form aninterferometric modulator, for example, an interferometric modulator ofthe general type illustrated in FIGS. 1 and 7. For example, the bottomlayer 151 in FIG. 12 can correspond to the substrate layer 20illustrated in FIGS. 1 and 7 and the upper layer 154 in FIG. 12 cancorrespond to the reflective layer 14 illustrated in FIGS. 1 and 7.Although not illustrated in FIG. 12, additional layers and materialsthat may be present in an interferometric modulator (e.g. optical stack16, deformable layer 34, etc.) can also be present in the methodillustrated by FIG. 12 at corresponding locations.

In the initial steps of manufacturing a MEMS device, a sacrificialmaterial 152 is provided over a substrate 151, and then a first metallayer 154 is then formed over the sacrificial material 152 to form astructure 150. The substrate 151 may be a transparent substrate such asglass or plastic and may have been subjected to prior preparationstep(s), e.g., cleaning. The substrate may also comprise additionallayers, such as those described above in the manufacture of aninterferometric modulator. In an embodiment, the underlying substratefurther comprises a conductive layer and/or an optical stack (notillustrated).

The sacrificial layer 152 is deposited at any desired thickness level.In an embodiment, the thickness of the sacrificial layer 152 is fromabout 0.1 micron to about 1 micron. The sacrificial layer 152 provides aspace filling material that can be easily etched away withoutundesirably affecting the other materials. In an embodiment, thesacrificial layer 152 is molybdenum. Other examples of suitablematerials for the sacrificial layer include polysilicon, amorphoussilicon, or photoresist. In a later step of manufacturing, thesacrificial layer 152 can be etched away to create a gap (e.g., the gap19) between a movable reflective layer and a dielectric layer or stack.

The materials used in the first metal layer (e.g., metal layer 154) canvary over a wide range to provide desired properties to the MEMS device.In an embodiment, the first metal layer comprises a highly reflectivematerial. Highly reflective materials are useful in optical MEMS devicesand can reflect light efficiently. In an embodiment, the first metallayer comprises a metal selected from the group consisting of aluminum,chromium, gold, silver, platinum, nickel, titanium, and tungsten. Othermetals can also be used in the first metal layer, including alloys andmixtures of metals. In an embodiment, the first metal layer comprisesaluminum. Highly reflective metals, including those described herein,can also form a mirror. In an embodiment, the first metal layercomprises a mirror.

Any known technique may be used to form the first metal layer 154,including deposition techniques such as physical vapor deposition (PVD),chemical vapor deposition (CVD), sputter deposition, and electroplating.The first metal layer 154 can then be masked and etched usingconventional masking and etching techniques, such as wet etching or dryetching. In an embodiment, the etching is performed by wet etching.Chemical wet etching allows for selective removal of the first metallayer by using a solution that dissolves the metal layer while themasked portion largely remains intact. As illustrated in FIG. 12B, afteretching the first metal layer 154, at least one first opening 155 isformed in the first metal layer 154 at the edges 153 of adjacent metalreflective pieces. The opening 155 exposes a first surface 152 a of thesacrificial material 152. The lateral size of the opening 155 (and thusthe CD loss) is typically related to the thickness of the first metallayer 154 because greater degrees of lateral etching rates typicallyresult when relatively thicker metal layers are used.

A second layer 156 of material is formed over the first surface 152 a ofthe sacrificial material 152, as illustrated in FIG. 12C. The materialsused in the second layer (e.g., layer 156) can vary over a wide range toprovide desired properties to the MEMS device, and may or may notinclude metal. In an embodiment, the second layer comprises a metal. Forexample, the second layer can comprise the same metallic material usedfor forming the first metal layer. In an embodiment, the second layercomprises a highly reflective material useful in the manufacture ofoptical MEMS devices. In an embodiment, the second layer comprises ametal selected from the group consisting of aluminum, chromium, gold,silver, platinum, nickel, titanium, and tungsten. Other metals can alsobe used in the second layer, including alloys and mixtures of metals. Inan embodiment, the second layer comprises aluminum. In an embodiment,the second layer comprises a mirror.

Highly reflective surfaces can also be formed using a metallic materialin the second layer that is different from that of the metallic materialin the first metal layer. In an embodiment, the second layer (e.g. layer156) comprises a metal that is different from the first metal layer.Non-metallic layers can also be used for the second layer. For example,where the first metal layer comprises aluminum, the second layer cancomprise a metallic material other than aluminum.

In other embodiments, the second layer (e.g. layer 156) can benon-metallic. Non-metallic materials in the second layer can provideadditional properties besides increased reflectivity to the first metallayer. In an embodiment, the second layer comprises a dielectricmaterial. For example, the second layer can comprise silicon. In anembodiment, the second layer comprises a polymer. Polymers can beproduced in large volumes and they provide a great variety of materialcharacteristics. Any polymer useful in the manufacture of optical MEMSdevices can be used in the methods described herein. For example, thepolymer can comprise polyethylene, polypropylene, polyester, polyamide,polyimide, etc., and combinations and copolymers thereof.

Any known technique may be used to form the second layer 156, includingthe deposition techniques discussed above. The thickness of the secondlayer 156 is preferably less than the thickness of the first metal layer154. The second layer 156 can then be masked and etched usingconventional masking and etching techniques, such as wet etching or dryetching. In an embodiment, the etching of the second layer 156 isperformed by wet etching. As illustrated in FIG. 12D, etching the secondlayer 156 creates at least one second opening 157, which also exposes atleast a portion 152 b of the first surface 152 a of the sacrificialmaterial 152. The second opening 157 has a smaller dimension, e.g.,lateral dimension, than the first opening 155 created during the etchingof the first metal layer 154. By narrowing the opening that wasinitially created during the first etching step, the total CD loss atthe mirror edges in the structure 150 is significantly reduced.

The second opening 157 has a smaller dimension than the first opening155 because, in part, the second layer 156 of material has a smallerthickness dimension than the first metal layer 154. Etching a thickerlayer sometimes involves more vigorous etching conditions andundercutting of the edge of the material can occur, further widening thegap between adjacent materials. The addition of the second layer 156 ofmaterial lessens the CD loss because the gap between the materials ofthe second layer 156 represented by the second opening 157 is less thanthe gap between the materials of the first metal layer 154 representedby the first opening 155.

In an embodiment, the first metal layer 154 and the second layer 156,together, have a surface area in contact with the sacrificial material152 that is larger than that of the first metal layer 154 alone. Asillustrated in FIG. 12C, the second layer 156 can be formed over otherareas in addition to the first surface 152 a of sacrificial material152, e.g., over the first metal layer 154, including the edges 153 ofthe first metal layer 154. In an embodiment, the method furthercomprises forming the second layer 156 over at least a portion of thefirst metal layer 154. In another embodiment, the second layer 156 isfurther formed over the entire area of the first metal layer 154, asillustrated, for example, in FIG. 12C.

Where the second layer 156 of material is formed over a portion of thefirst metal layer 154 and the first surface 152 a of exposed sacrificialmaterial 152, and then etched to create a second opening 157, the secondlayer 156 of material effectively provides a winged extension 156 a tothe first metal layer 154. As shown in FIG. 12E, the sacrificial layer152 is removed leaving a surface comprising the first metal layer 154and the winged extension 156 a of the second layer 156 facing thesubstrate 151 and separated from the substrate 151 by a gap 159. Supportposts (not illustrated), e.g. support posts represented in FIGS. 1 and7, or other structures that separate two layers can be put in place tomaintain the spacing of the gap 159 between the substrate 151 and thefirst metal layer 154. The gap 159 in FIG. 12E can correspond to the gap19 in FIG. 1.

In an embodiment, the surface comprising the first metal layer 154 andthe winged extension 156 a of the second layer 156 that faces thesubstrate 151 is a highly reflective surface, such as a mirror. The CDloss between adjacent mirrors is significantly reduced because the CDloss has been reduced by the winged extension 156 a, thus creating aMEMS device having substantially increased brightness due, in part, tothe high reflectivity of the mirrors and the large surface area.

The methods described herein can further be used in the formation ofelectromechanical devices, such as electromechanical devices comprisinghighly reflective surfaces. In an embodiment, an electromechanicaldevice comprises a mirror. The mirror can be formed using the methodsdescribed in FIGS. 12A through 12E. In an embodiment, the mirrorcomprises a core portion 154 having an exposed reflective surface 154 eand an overlaying mirror extension portion 156 a also having an exposedreflective surface 156 e. As illustrated in the embodiment shown in FIG.12E, the exposed reflective surface 154 e of the core portion 154 andthe exposed reflective surface 156 e of the mirror extension portion 156a are co-planar.

In an embodiment, and discussed in further detail below, the overlayingmirror extension portion 156 a has at least a portion of its thicknessdimension smaller than the thickness dimension of the core portion 154.The core portion 154 of the mirror can comprise a homogenous material.In an embodiment, the core portion 154 of the mirror is a unitarymaterial. In an embodiment, the core portion 154 of the mirror comprisesa single layer.

The methods of fabricating a MEMS device described herein are notlimited to providing a first metal layer and a second layer. Additionallayers can be provided without departing from the scope of the teachingsherein. In an embodiment (not illustrated), the method of fabricating aMEMS device further comprises forming a third layer over the portion 152b of the first surface 152 a of the sacrificial material 152 in a mannersimilar to that depicted in FIG. 12 and described above. In anembodiment, the third layer has a smaller thickness dimension than thesecond layer 156. The third layer can then be patterned and etched toform at least one third opening in the third layer, whereby the thirdopening exposes at least a portion of the portion 152 b of the firstsurface 152 a of the sacrificial material 152. Preferably, the thirdopening has a smaller dimension, e.g., lateral dimension, than thesecond opening 157, thus further reducing CD loss.

Multiple formations of additional layers with decreasingly smallerthickness dimensions can further compensate for the CD loss that occursduring fabrication of MEMS devices. For example, in an embodiment, afourth layer is formed over a portion of the first surface 152 a ofsacrificial material 152 with a smaller thickness dimension than theprevious layer. After similar pattern and etching processing stepspreviously performed on the second and third layers are performed on thefourth layer, a fourth opening exposes at least a portion of the firstsurface 152 a of sacrificial material 152, whereby the fourth openinghas a smaller dimension, e.g., lateral dimension, than the thirdopening. The number of layers useful in the methods described herein isnot limited. For example, the methods of fabricating a MEMS device maycomprise manufacturing steps using anywhere from two to ten layers, ormore. Additional layers can be formed using similar depositiontechniques as described above with regards to the second layer, suchthat each additional layer formed has a smaller thickness dimension thanthe layer deposited before it.

Thus, electrochemical devices made in accordance with the methodsdescribed can optionally have further overlaying mirror extensionportions. In an embodiment, the electrochemical devices described hereincomprise a second overlaying mirror extension portion having an exposedreflective surface, wherein the exposed reflective surface of the secondoverlaying mirror extension is co-planar with the exposed reflectivesurfaces of the core portion and the first mirror extension portion.Additional overlaying mirror extension portions are furthercontemplated. As described in greater detail below, the secondoverlaying mirror extension portion can have at least a portion of itsthickness dimension smaller than the thickness dimension of the coreportion. Furthermore, the optional second overlaying mirror extensionportion can also have at least a portion of its thickness dimensionsmaller than the thickness dimension of the first overlaying mirrorextension portion.

By fabricating a MEMS device according to the method described in FIG.12, CD loss is significantly reduced, even while using relatively thickmirrors. For example, CD loss can be less than 7 microns on a mirrorhaving a thickness of greater than about 0.75 microns. Additionally, CDloss can be less than 7 microns on a mirror having a thickness greaterthan about 1 micron.

Another method of fabricating a MEMS device may be used that can alsoprovide winged extensions to the edge of a metal layer, thereby reducingCD loss. The initial steps of fabricating the MEMS device according tothis embodiment are similar to those disclosed above with respect to theembodiment shown in FIG. 12. For example, a first metal layer is formedover a sacrificial layer and then etched to provide and at least oneopening in the first metal layer. However, the winged extensions areprovided to the first metal layer in a different manner. Each of the atleast one openings separate two or more portions (e.g. mirrors) of thefirst metal layer. A second layer can be deposited, masked, and etchedfrom one side of the opening so as to provide a winged extension to oneof the mirrors. Then, a third layer can be deposited, masked, and etchedfrom the other side of the opening so as to provide a winged extensionto the other mirror layer. The lateral dimension of the opening can besignificantly reduced by this method of offset patterning and etchingusing precision masking and etching techniques. As the lateral dimensionof the opening is reduced, CD loss in the resulting MEMS device is alsoreduced.

FIGS. 13A through 13F illustrate this embodiment of the processing stepsused in the manufacture of a MEMS device. The structure 200 illustratedin FIG. 13 can be used to form an interferometric modulator, forexample, an interferometric modulator of the general type illustrated inFIGS. 1 and 7. For example, the bottom layer 201 in FIG. 13 cancorrespond to the substrate layer 20 illustrated in FIGS. 1 and 7 andthe upper layer 204 in FIG. 13 can correspond to the reflective layer 14illustrated in FIGS. 1 and 7. Although not illustrated in FIG. 13,additional layers and materials that may be present in aninterferometric modulator (e.g. optical stack 16, deformable layer 34,etc.) can also present in the method illustrated by FIG. 13 atcorresponding locations.

The fabrication methods illustrated in FIGS. 13A through 13F optionallycomprise forming an etch stop after each deposition. While using etchstops after depositions requires additional processing steps, suchmethods can further significantly decrease CD loss because the final CDloss dimension is determined by the edges of the mirror from differentdeposition steps.

In the initial steps of manufacturing a MEMS device, a sacrificialmaterial 202 is provided over a substrate 201, and then a first metallayer 204 is then formed over the sacrificial material 202. Thematerials used for each of the substrate 201, sacrificial material 202,and first metal layer 204 can vary. Preferably, the type of materialsused in each of the substrate 201, sacrificial material 202, and firstmetal layer 204 in this embodiment are similar to those described abovein the corresponding portions of FIG. 12 (e.g., substrate 151,sacrificial material 152, and first metal layer 154, respectively).

Any known technique may be used to form the first metal layer 204,including deposition techniques such as PVD, CVD, sputtering, andelectroplating. The first metal layer 204 is then masked and etchedusing either wet etching or dry etching techniques. FIG. 13A illustratesa structure 200 after initial formation of the first metal layer andafter the initial masking and etching steps. A masking layer 214 remainsover the first metal layer, and can optionally be removed.

Removal of the masking layer 214 may depend on subsequent etchants usedin subsequent processing steps. Etchants are selected based on thematerial to be etched, and each etchant can affect materialsdifferently. For example, the masking layer can be removed if adifferent etchant is to be used in later processing steps that does notaffect the first metal layer. In some embodiments, the masking layer 214can remain with further layer formations. For example, if subsequentlayers comprise a similar material to the first metal layer that willalso be etched with a similar etchant, the masking layer can remain toprotect the first metal layer from the etchant.

In an embodiment, the etching is performed by wet etching. After etchingthe first metal layer 204, at least one first opening 205 is formed inthe first metal layer 204 to form structure 200, as illustrated in FIG.13A. The opening 205 exposes a first surface 202 a of the sacrificialmaterial 202. The size of the opening 205 is related to the thickness ofthe first metal layer 204 because greater degrees of etching aretypically involved when thicker layers are used.

As illustrated in FIG. 13B, a second layer 206 is then formed over thefirst surface area 202 a of the sacrificial material 202. After thesecond layer is formed, a masking layer 216 is further formed over thesecond layer 206 and over a portion of the first surface area 202 a ofthe sacrificial material 202 to thereby form an unmasked portion 226 ofthe second layer 206 over the first surface 202 a from FIG. 13A.

As illustrated in FIG. 13C, the unmasked portion 226 of the second layer206 is then etched, using conventional etching techniques, to form atleast one opening 207 in the second layer 206. In an embodiment, theetching is performed using a wet etch technique. The at least oneopening 207 in the second layer exposes a second surface area 202 b ofthe sacrificial material 202. The second surface area 202 b of thesacrificial material 202 is smaller than the first surface area 202 a ofthe sacrificial material 202, illustrated in FIG. 13A. The portion ofthe second layer 206 under the mask 216 is protected from the etchingstep and remains with the structure. Following the etching step, themasking layer 216 can optionally be removed. Removal of the maskinglayer 216 may depend on the subsequent etching and processing steps tobe used, as discussed above.

As illustrated in FIG. 13C, at least a portion of the second layer 206forms a winged extension 206 a of the first metal layer 204 from oneside and serves to at least partially fill and therefore, decrease thesize of the opening 207. The materials used for the second layer 206 canvary, e.g., metallic or non-metallic. Preferably, the materials used forthe second layer 206 in this embodiment are similar to those describedabove in the corresponding portions of FIG. 12 (e.g., second layer 156).

The second layer 206 can be formed over areas other than the firstsurface 202 a of sacrificial material 202. In an embodiment (and asshown in FIG. 13), the second layer 206 is further formed over at leasta portion of the first metal layer 204 in addition to being formed overthe first surface 202 a of sacrificial material 202. In an embodiment,the second layer 206 has a smaller thickness dimension than the firstmetal layer 204.

FIG. 13D further illustrates another embodiment, wherein the methodfurther comprises forming a third layer 208 over at least a portion ofthe exposed second surface area 202 b (from FIG. 12C) of the sacrificialmaterial 202. It is also contemplated that the third layer 208 can befurther formed over at least a portion of the first metal layer 204and/or at least a portion of the second layer 206. An additional maskinglayer 218 is formed over the third layer 208 and over a portion of thesecond surface area 202 b of the sacrificial material 202 forming anunmasked portion 228 of the third layer 208 over the second surface area202 b.

As illustrated in FIG. 13E, the unmasked portion 228 of the third layer208 is then etched, such as by wet etching, to form at least one opening209 in the third layer 208. The portion of the third layer 208 under themask 218 is protected from the etching step and remains. Masking layer218 can, in some embodiments, subsequently be removed. After etching, atleast a portion of the third layer 208 forms a winged extension 208 a ofthe first metal layer 204 from the opposite side of the opening that theother winged extension 206 a filled in. Thus, winged extension 208 aalso serves to at least partially fill and therefore, decrease the sizeof the opening 209.

The materials used for the third layer 208 can vary, e.g., metallic ornon-metallic. Preferably, the materials used for the third layer 208 inthis embodiment are similar to those described above in the second layer156 of FIG. 12. The at least one opening 209 in the third layer 208exposes a third surface area 202 c of the sacrificial material 202. Thethird surface area 202 c of the sacrificial material 202 is smaller thanthe second surface area 202 b of the sacrificial material.

The CD loss in a mirror layer is minimized using the fabrication methodsshown in FIGS. 13A through 13E. FIG. 13C shows that the second layer 206minimized the CD loss of the in the first metal layer 204 from one edge,e.g., one mirror; while FIG. 13E shows that the third layer 208minimized CD loss of the first metal layer 204 from the other edge,e.g., mirror. By alternating the formation of additional layers on theedges of the mirrors using etch stops, e.g., offset patterning, CD lossis significantly reduced, even while using relatively thick mirrors. Forexample, CD loss can be less than 7 microns on a mirror having athickness of greater than about 0.75 microns. Additionally, CD loss canbe less than 7 microns on a mirror having a thickness greater than about1 micron.

Afterward, the sacrificial material can be removed from the structure.FIG. 13F shows the removal of the sacrificial layer 202 to form a gap210. Support posts (not illustrated), e.g. support posts as representedin FIGS. 1 and 7, can be put in place to maintain the spacing of the gap210 between the substrate 201 and the first metal layer 204. The gap 210in FIG. 13F can correspond to the gap 19 in FIG. 1. While the maskinglayers 214, 216, and 218 are not illustrated in FIG. 13F, in someembodiments, any one or more of masking layers can be present. The CDloss between adjacent mirrors in the first metal layer 204 has beensignificantly reduced by winged extensions 206 a and 208 a, which weresupplied from the second layer 206 and third layer 208, respectively.

Subsequent layers can be further provided in an analogous fashion. Thematerials used in any additional layer (e.g. fourth layer, fifth layer,sixth layer, etc.) can vary over a wide range to provide desiredproperties to the MEMS device. For example, additional layers can beeither metallic or non-metallic, as discussed above. Suitable metalsinclude aluminum, chromium, gold, silver, platinum, nickel, titanium,and tungsten. Additional layers, via winged extensions, such as 206 aand 208 a can provide increased reflectivity to the first metal layer.It is further contemplated that the winged extensions can benon-metallic and provide properties other than reflectivity.

The thicknesses of the first metal layer, the second layer, and anysubsequent layers in all of the embodiments described herein can varyand those having ordinary skill in the art, guided by the disclosureherein, can provide layers with appropriate thicknesses using variousdeposition techniques. Selection of appropriate thickness can depend onthe desired flexibility of the layer in the end use of the product.

In an embodiment, the first metal layer in any embodiment has athickness in the range of about 0.1 microns to about 10 microns. In anembodiment, the first metal layer has a thickness in the range of about0.75 microns to about 3 microns. In an embodiment, the first metal layerhas a thickness in the range of about 1 micron to about 2.5 microns. Inan embodiment, the first metal layer has a thickness of about 1 micron.In an embodiment, the first metal layer has a thickness of about 2microns. In an embodiment, the first metal layer has a thickness ofabout 3 microns.

Preferably, the second layer has a smaller thickness dimension than thefirst metal layer. In an embodiment, the second layer in any embodimenthas a thickness in the range of about 0.01 microns to about 2 microns.In an embodiment, the second layer has a thickness in the range of about0.02 microns to about 0.2 microns. In an embodiment, the second layerhas a thickness in the range of about 0.03 microns to about 0.1 microns.

Where additional layers are formed after the second layer in any of theembodiments described herein, the thickness of those layers can be inabout a similar range as for the second layer. However, it ispreferable, though not required, that the thicknesses of the additionallayers be less than the thickness of the layer that had been formedbefore it.

In embodiments where the first metal layer and the second layer,together, comprise a mirror, e.g., where second layer comprises the samemetal that is used in the first metal layer, the combined thicknesses ofthe first metal layer and the second layer equal the overall thicknessof the mirror, in the absence of the wing portion. One can individuallyselect the thickness of the first metal layer and the second layer tocreate a mirror having a desired final thickness. For example, a firstmetal layer having a thickness of 1.9 microns can be combined with asecond layer having a thickness of 0.1 microns to create a mirror havinga total overall thickness of 2 microns, not counting the thickness ofthe wing portion.

Another embodiment provides an interferometric modulator fabricated bythe methods described herein, including the methods illustrated by FIGS.12A through 12E and FIGS. 13A through 13F. For example, aninterferometric modulator can be fabricated using the flow diagram shownin FIG. 8. The MEMS devices fabricated by the methods described hereincomprise mirror layers with high reflectivity because the methodsminimize the amount of CD loss in the mirrors.

Another embodiment provides an interferometric modulator comprising aplurality of mirrors, wherein the horizontal spacing between theplurality of mirrors is minimized. In an embodiment, an interferometricmodulator comprises a substrate, an optical layer patterned into rows, amechanical layer patterned into columns, a mirror layer separated fromthe optical layer by a vertical gap and comprising a plurality ofmirrors, wherein the mirror layer has a thickness greater than or equalto about 0.75 microns; and wherein the plurality of mirrors areseparated from one another by a horizontal gap that is less than orequal to about 7 microns. Such dimensions of mirror thickness andhorizontal gap spacing are made possible using the methods describedherein.

The interferometric modulators described herein provide minimal gapspacing between mirrors, wherein the mirror thickness is greater thanabout 0.75 microns. In an embodiment, the gap spacing between mirrors isless than or equal to about 7 microns, wherein the mirror thickness isgreater than about 1 micron. In an embodiment, the gap spacing betweenmirrors is less than or equal to about 7 microns, wherein the mirrorthickness is greater than about 1.5 microns.

Mirror thickness can be altered by adjusting the amount of metal that isinitially deposited on the sacrificial material. In an embodiment, themirror has a thickness in the range of about 0.75 microns to about 3microns. In an embodiment, the mirror has a thickness in the range ofabout 1 micron to about 2.5 microns. In an embodiment, the mirror has athickness in the range of about 1.5 microns to about 2 microns. Thickermirrors provide increased rigidity and are less likely to warp duringuse of the MEMS device.

In addition to minimized horizontal gap spacing between mirrors having athickness greater than about 0.75 microns, the plurality of mirrors canalso have a relatively large surface area. Surface area of the mirrorcan be increased by providing winged extensions to the first metallayer, wherein the winged extensions are also made from a highlyreflective material. The methods of fabricating a MEMS device describedherein provide the unique combination of relatively large mirrorthickness, relatively high surface area of the mirror layer, and low CDloss between adjacent mirrors. In an embodiment, each of the pluralityof mirrors has at least one longitudinal or lateral dimension greaterthan about 20 microns. In an embodiment, each of the plurality ofmirrors has at least one longitudinal or lateral dimension greater thanabout 25 microns. In an embodiment, each of the plurality of mirrors hasat least one longitudinal or lateral dimension greater than about 30microns.

In an embodiment, the mirror has tapered edges. Tapered edges can beprovided by forming a first metal layer that is tapered, e.g.trapezoidal in cross-sectional shape. Tapered edges can also be formedby shaping the winged extensions during subsequent layer formations.FIG. 14 is an embodiment of a structure 300 having a reflective layerwith tapered edges. A sacrificial layer 302 is formed on top of asubstrate 301 and a mirror layer 304 is subsequently deposited on top ofthe sacrificial material 302. The mirror layer 304 is provided with atapered edge 306. During the course of etching the mirror layer 304, thedegree of the taper is lessened over time, as the edges of the taper 306have a smaller thickness than the interior portion of the taper 306,causing undercutting at the mirror edge. After the mirror layer 304 isetched, the tapered edge 306 is eroded slightly to form the final taper308. The thickness of the tapered edges varies along the length of thetaper. In an embodiment, the tapered edges have a thickness of about0.01 microns to about 2 microns at any point in the taper. In anembodiment, the tapered edges have a thickness of about 0.02 microns toabout 1 micron at any point in the taper.

In an embodiment, the MEMS fabrications steps described herein can beused to make an interferometric modulator. In an embodiment, theinterferometric modulator further includes a display. In someembodiments, the interferometric modulator includes a processor. Theprocess can be configured to communicate with the display. Additionally,the processor can be configured to process image data. In someembodiments, the interferometric modulator includes a memory device thatis configured to communicate with said processor. In an embodiment, theinterferometric modulator further includes a driver circuit configuredto send at least one signal to the display.

In an embodiment, the interferometric modulator further includes acontroller configured to send at least a portion of the image data tothe driver circuit. In an embodiment, the interferometric modulatorfurther includes an image source module configured to send said imagedata to said processor. In an embodiment, the image source moduleincludes at least one of a receiver, transceiver, and transmitter. In anembodiment, the interferometric modulator further includes an inputdevice configured to receive input data and to communicate said inputdata to said processor.

1. A method of fabricating an electromechanical systems device,comprising: providing a sacrificial material; forming a first metallayer over the sacrificial material; etching the first metal layer toform at least one first opening in the first metal layer to therebyexpose a first surface of the sacrificial material; forming a secondlayer over the first surface of the sacrificial material, wherein thesecond layer has a smaller thickness dimension than the first metallayer; and etching the second layer to form at least one second openingin the second layer to thereby expose at least a portion of the firstsurface of the sacrificial material, wherein the second opening has asmaller dimension than the first opening.
 2. The method according toclaim 1, wherein etching the first metal layer comprises wet-etching. 3.The method according to claim 1, wherein etching the second layercomprises wet-etching.
 4. The method according to claim 1, furthercomprising forming the second layer over at least a portion of the firstmetal layer.
 5. The method according to claim 1, wherein the first metallayer comprises a mirror.
 6. The method according to claim 1, whereinthe second layer comprises a metal.
 7. The method according to claim 6,wherein the second layer comprises a mirror.
 8. The method according toclaim 6, wherein the first metal layer and the second layer comprise thesame metal.
 9. The method according to claim 8, wherein the first metallayer and the second layer together have a surface area in contact withthe sacrificial material that is larger than that of the first metallayer alone.
 10. The method according to claim 1, wherein the firstmetal layer has a thickness in the range of about 0.75 microns to about3 microns.
 11. The method according to claim 1, wherein the second layerhas a thickness in the range of about 0.02 microns to about 0.2 microns.12. The method according to claim 1, wherein the sacrificial material isprovided on an underlying substrate.
 13. The method according to claim12, wherein the underlying substrate comprises a conductive layer. 14.The method according to claim 13, further comprising removing thesacrificial material to thereby form a gap between the first metal layerand the conductive layer.
 15. A method for fabricating anelectromechanical systems device, comprising: providing a sacrificialmaterial; forming a first metal layer over the sacrificial material;etching the first metal layer to form at least one opening in the firstmetal layer to thereby expose a first surface area of the sacrificialmaterial; forming a second layer over the first surface area of thesacrificial material, the second layer having a smaller thicknessdimension than the first metal layer; forming a first masking layer overthe second layer and over a portion of the first surface area of thesacrificial material to thereby form an unmasked portion of the secondlayer over the first surface area; and etching the unmasked portion ofthe second layer to form at least one opening in the second layer tothereby expose a second surface area of the sacrificial material,wherein the resulting exposed second surface area of the sacrificialmaterial is smaller than the first surface area of the sacrificialmaterial.
 16. The method according to claim 15, further comprising thestep of removing the first masking layer.
 17. The method according toclaim 15, wherein etching the first metal layer comprises wet-etching.18. The method according to claim 15, wherein etching the second layercomprises wet-etching.
 19. The method according to claim 15, wherein thesecond layer is further formed over at least a portion of the firstmetal layer.
 20. The method according to claim 15, wherein the firstmetal layer comprises a mirror.
 21. The method according to claim 15,wherein the second layer comprises a metal.
 22. The method according toclaim 21, wherein the second layer is a mirror layer.
 23. The methodaccording to claim 15, wherein first metal layer has a thickness in therange of about 0.75 microns to about 3 microns.
 24. The method accordingto claim 15, wherein second layer has a thickness in the range of about0.02 microns to about 0.2 microns.
 25. The method according to claim 15,further comprising: forming a third layer over at least a portion of theexposed second surface area of the sacrificial material; forming asecond masking layer over the third layer and over a portion of thesecond surface area of the sacrificial material to thereby form anunmasked portion of the third layer over the second surface area; andetching the unmasked portion of the third layer to form at least oneopening in the third layer to thereby expose a third surface area of thesacrificial material, wherein the third surface area of the sacrificialmaterial is smaller than the second surface area of the sacrificialmaterial.
 26. The method according to claim 25, wherein the third layerhas a thickness in the range of about 0.02 microns to about 0.2 microns.27. An interferometric modulator comprising: a substrate; an opticallayer patterned into rows; a mechanical layer patterned into columns; amirror layer separated from the optical layer by a vertical gap andcomprising a plurality of mirrors; wherein the mirror layer has athickness greater than or equal to about 0.75 microns; and wherein theplurality of mirrors are separated from one another by a horizontal gapthat is less than or equal to about 7 microns.
 28. The interferometricmodulator of claim 27, wherein each of the plurality of mirrors has alongitudinal or lateral dimension greater than about 25 microns.
 29. Theinterferometric modulator of claim 27, wherein the mirror has athickness in the range of about 0.75 microns to about 3 microns.
 30. Theinterferometric modulator of claim 27, wherein the mirror has taperededges.
 31. The interferometric modulator of claim 27, wherein thetapered edges have a thickness of about 0.02 microns to about 2 microns.32. The interferometric modulator of claim 27, further comprising: adisplay; a processor that is configured to communicate with saiddisplay, said processor being configured to process image data; and amemory device that is configured to communicate with said processor. 33.The interferometric modulator of claim 32, further comprising a drivercircuit configured to send at least one signal to the display.
 34. Theinterferometric modulator of claim 33, further comprising a controllerconfigured to send at least a portion of the image data to the drivercircuit.
 35. The interferometric modulator of claim 32, furthercomprising an image source module configured to send said image data tosaid processor.
 36. The interferometric modulator of claim 35, whereinthe image source module comprises at least one of a receiver,transceiver, and transmitter.
 37. The interferometric modulator of claim32, further comprising an input device configured to receive input dataand to communicate said input data to said processor.
 38. Anelectromechanical device comprising a mirror, wherein the mirrorcomprises a core portion having an exposed reflective surface and anoverlaying mirror extension portion having an exposed reflectivesurface; wherein the exposed reflective surface of the core portion andthe exposed reflective surface of the mirror extension portion areco-planar.
 39. The electromechanical device of claim 38, wherein theoverlaying mirror extension portion has at least a portion of itsthickness dimension smaller than the thickness dimension of the coreportion.
 40. The electromechanical device of claim 38, wherein the coreportion comprises a homogenous material.
 41. The electromechanicaldevice of claim 38, wherein the core portion comprises a single layer.42. The electromechanical device of claim 38, further comprising asecond overlaying mirror extension portion having an exposed reflectivesurface, wherein the exposed reflective surface of the second overlayingmirror extension portion is co-planar with the exposed reflectivesurfaces of the core portion and the first mirror extension portion. 43.The electromechanical device of claim 42, wherein the second overlayingmirror extension portion has at least a portion of its thicknessdimension smaller than the thickness dimension of the core portion. 44.The electromechanical device of claim 42, wherein the second overlayingmirror extension portion has at least a portion of its thicknessdimension smaller than the thickness dimension of the first overlayingmirror extension portion.